City of Houston Water Flood Hazards Map

City of Houston Map Shows Extent of Water Flood Hazards for Many Smaller Ditches, Streams

If you fear flooding from a small stream and you live inside the Houston City limits, this is THE resource for you. It’s the City’s Water Flood Hazards Viewer. And it’s incredibly detailed. It even shows the extent of flood hazards around many unnamed temporary streams and man-made ditches.

Water/Flood Hazards within the City of Houston

The image above shows Ben’s Branch where it cuts across the Harris/Montgomery County Line. Many homes and businesses in this area flooded during Harvey and Imelda despite being three miles from the nearest river.

The different shadings and cross-hatchings represent floodways, 100-year and 500-year floodplains. I checked around neighborhoods near me where I was familiar with the extent of flooding. The map seems to be very accurate. I only found one or two properties that I would have reclassified.

Impressive Gallery of Base Maps and Other Features

The City’s Water Flood Hazards site contains an impressive gallery of 25 different base map options. They range from customary street maps and satellite views to topographic maps and more.

You can also measure distance and direction with the built in tools. And unlike many such maps, this one contains built in drawing tools that let you make notations and share them with your friends or associates.

Of course, it includes information on flood zones near major rivers, too. It even includes information on hurricane evacuation zones near the coast.

You can see the outlines of Harris County in the extent of the waterways. The bright areas near the coast represent hurricane evacuation zones and zip codes.

Incredible Flexibility

Because the system is built on a geographic information system database, you can even add data from other databases. That makes the map reflect additional types of information. But warning: this can get very technical.

Great Tool for Research

If you’re considering buying a property or flood insurance, and you live in the City or Harris County, this is definitely a map you should bookmark. The Water Flood Hazards Map is an incredible research tool. For ease of future reference, I have listed the map on the Links page of this website under Floodplain Maps and Elevation.

Posted by Bob Rehak on 9/6/2020

1104 Days after Hurricane Harvey

Tetra Tech Study Provides Clues To Possible Mouth Bar Dredging Strategies

FEMA has agreed to dredge another million cubic yards from the the area near the San Jacinto West Fork Mouth Bar. A report produced for the City of Houston by Tetra Tech helped convince FEMA. The report relied on sonar, LIDAR, and core sample data to estimate the total volume of sand deposited by Hurricane Harvey in that area: approximately 1.4 million cubic yards.

Need for Ruthless Efficiency

While another million cubic yards may sound like a lot, the area is huge. Dredging the whole 4.3 million square yard area would add only about 8 inches of water depth and leave an underwater mesa between the West Fork and the Lake. According to local geologists Tim Garfield and RD Kissling, who have studied the problem extensively, that would create a sediment trap that accelerates accumulation of sand from future storms.

So, what to do?

Three Strategies Discussed to Date

Those close to the project have discussed several strategies to date.

  • The Corps’ initial strategy: Dredge upstream from the mouth bar. They said 1D modeling showed that would accelerate water flowing into mouth bar and give it the velocity needed to push sand from the mouth bar farther out into the lake.
  • Another strategy: dredge downstream from the mouth bar and let the river push the mouth bar into the dredged area.
  • A third strategy: reconnect the river and the lake with a narrow channel that accelerated the flow of water and carried suspended sediment out into the broader lake south of the 1960 bridge.

2019 Tetra Tech Report

Stephen Costello, the City’s flood czar, says that new survey and modeling work has yet to be completed. That will ultimately determine where new dredging happens. However, he also added that consulting Tetra Tech’s exhibits would help provide clues as to where dredging might be most effective, based on knowledge accumulated to date.

The first chart in Appendix A showed the coring locations and transects (survey lines) of the lake’s bottom profile.
The second chart shows what they found in various coring locations. The feet indicate the thickness of the top layer.

Composition of the core samples provides clues as to what was laid down when. Sand (the yellow dots) is generally laid down during floods which have the energy to transport the heavy particles. However, clay and silt (the green and blue dots) are smaller. So they tend to drop out of suspension when water is calmer.

Finding sand above silt in a core sample indicates that a storm like Harvey likely laid down the sand.

The third chart is the most crucial. It’s a difference map that shows areas of deposition and scour pre- and post-Harvey. This shows two things: where most sediment fell out of suspension and where the main flow of the river tried to churn a path through the mounting muck.

From the difference map above, you can see that the river tried to scour its way through the sediment along a path from LH-16 to LH-21 to LH-23. You can see another area of scour to the far right from LH-15 to LH-25 to LH-26.

Where River Flowed Before Lake Was Impounded

Interestingly, the area of scour to the left follows the river’s relic channel.

San Jacinto River map before Lake Houston was impounded

Note how the West Fork hugged what is now Atascocita Point – the thumb of high land that sticks up in the Tetra Tech illustrations.

Harnessing Natural Energy of the River

From the third and fourth illustrations above, one might conclude that excavating a channel near Atascocita Point represents the best way to harness the natural energy of the river. That’s the shortest channel where scour is deepest.

Given the million cubic yard limit, that path also represents a chance to dig the deepest, widest channel possible within the budget. When technicians compiled the difference map above, most of that path was already at or below its 2011 level.

500,000 square-yard path outlined in yellow would let dredgers excavate six feet. Average bottom depth is already 5.5 feet in that area.

Following that path also lets you funnel future sediment through the FM1960 causeway and disperse it out into the wider, deeper lake.

Next Steps and Timing

At this point, we don’t know what Imelda did to this area. Imelda struck shortly after the Army Corps completed its post-dredging survey in this area last year.

Before the additional dredging can begin, several things must happen.

  • Completion of a new survey
  • Model different scenarios
  • Identify best strategy
  • Locate suitable placement area
  • Compile scope of work
  • Bid job
  • Mobilize

Based on past experience, that could take months to a year or more. It took 13 months after Harvey for the Corps to put equipment in the water for its Emergency West Fork Dredging Project. However, we don’t have as many unknowns this time.

Posted by Bob Rehak on 9/5/2020

1103 Days since Hurricane Harvey

HCFCD to Begin Next Phase of Ben’s Branch Clean Out in October

Harris County Flood Control District (HCFCD) and its contractors met today with representatives of Kings Forest, the Bear Branch Trail Association and Kingwood Service Association to discuss the next phase of the Ben’s Branch clean out. On the south, the project lies entirely within the Creekwood Nature Area between Kingwood High School and the old H-E-B shopping center. Rocky Woods Drive forms the northern extent of the project. This will extend northward the work already completed south of Kingwood Drive.

Project extent outlined in red. Kingwood High School is at bottom center; Town Center on right; Kings Forest on left and Bear Branch at top of frame.

Maintenance Objective: Restore Conveyance

The objective: to restore conveyance of Ben’s Branch and reduce potential for flooding in Kings Forest, Bear Branch, the Kingwood High School, and Kingwood Town Center.

The ground these men are standing on is all deposited sediment that needs removal to restore conveyance. The original channel bank is the higher slope behind them. The other side has a similar problem.

Sediment has restricted the flow of the channel gradually during the last three decades. It now contributes to flooding.

Tucked into the tree line on either side of Bens Branch, you can see the maintenance roads that formed the top of the original banks.

During Harvey, many homes on both sides of the stream flooded. Many also flooded again during Imelda.

Walking along the creek today, the first thing one notices is a craggy channel with sides that seem to have slumped into the stream. Flood control surveys, however, show that is not the case. The channel filled with sediment. Then the stream eroded down again through the accumulated sediment.

Continual cycles of deposition and erosion have clogged, deformed and narrowed the creek.

Approximately 15,000 Cubic Yards of Sediment To Be Removed

Getting the channel back to its original state will require removal of approximately 15,000 cubic yards of sediment. However, engineers have not yet determined the exact number.

The scope of work will include replacement of damaged drain pipes that carry water to the ditch.

The job is still in its planning stages. Actual dirt work should begin sometime in October.

Funded with Help from USDA NRCS

A grant from the USDA Natural Resources Conservation Service (NRCS) will help HCFCD fund the project. Contractors will haul the excavated dirt to nearby TCEQ-approved landfill sites outside of the .02% annual chance (500-year) flood plain.

Looking north. The project will NOT extend into the natural portion of Ben’s Branch at the top of the frame, near Rocky Woods Drive. It will affect only the man-made portion of the channel.

Downstream, the project will stop at Kingwood Drive. Note below how the channel under the Kingwood Drive bridge is virtually twice as large as the channel in the foreground.

Looking south over Ben’s Branch toward Kingwood Drive and the portion of Ben’s Branch restored earlier this year. Note how constricted the channel in the foreground is.

When complete, this project should make the channel north of Kingwood Drive as wide as it is south of Kingwood Drive. It’s all about getting the channel back to its designed carrying capacity.

An exact timetable for the project is not yet available, but it will take several months.

Posted by Bob Rehak on 9/4/2020

1102 Days after Hurricane Harvey